A report by Commonwealth and Victoria Police in 1968 didn’t officially determine a cause of death, but did conclude that "there has been no indication that the disappearance of the late Mr Holt was anything other than accidental."

With both Cold War paranoia and civil unrest over the Vietnam War at their peak, conspiracy theories quickly began to take hold.

A Chinese Spy

One of the more outlandish theories, popularised by a British novelist, is that Holt had been a spy for the Chinese for more than 30 years, was picked up by a submarine and taken back to China.

Holt’s wife countered this particularly wild theory by noting that he “didn’t even like Chinese cooking.”

An Assassination

Just the previous year, the leader of the Labor opposition Arthur Calwell had survived an assassination attempt by a student who disagreed with his anti-war stance.

After Holt’s death, the police found what looked like a bullet hole in a window of the Prime Minister’s office, bolstering the theory that he may have been murdered.

A Botched Kidnapping

A book by Scott Cooper claims that Holt was killed accidentally in a botched kidnapping that was prompted by his plan to increase troops to Vietnam.

Other outlandish theories suggest Holt faked his death or was taken by a UFO.

Harold Holt’s former press secretary Tony Eggleton maintains that the PM’s demise was nothing more than a tragic accident.

"Basically, I think that people just find it very hard to accept that a prime minister can go for a swim on a Sunday afternoon, like anyone else, and end up misjudging the situation and drowning.”